Titanium-containing metals are of great interest to the aerospace industry because they have low densities, low thermal expansion coefficients, and high structural strengths. Parts made from titanium-containing metals are lightweight, and can withstand high thermal stresses and high physical loads.
In some applications, it is desirable to deposit a metallic coating onto the surface of the part. However, the part rapidly oxidizes when exposed to oxygen to create an oxide layer that is electrically and chemically passive in nature. The presence of this passive oxide layer severely inhibits the chemical bonding that takes place between the metallic coating and the part. As a result, it is extremely difficult to deposit an adherent metal coating onto the part. Even when the metallic coating is successfully deposited onto the oxide layer of the part, adhesion tends to be poor. Consequently, the metallic coating is of little value since it can easily be removed from the surface of the part by bending, peeling and/or scratching.
Aggressive pretreatments, such as grit blasting and/or the use of harsh etchants (i.e., hydrofluoric acid or chrome-based chemicals), are commonly used to remove the passive oxide layer from the surface of the part prior to depositing the metallic coating. However, these aggressive pretreatments cannot be used for precision electronic aerospace parts because such parts have tight tolerances and prescribed surface finishes. Moreover, the harsh etchants are harmful to humans and to the environment.
U.S. Pat. No. 5,464,524 discloses a plating method for a nickel-titanium alloy member that comprises the steps of: (a) subjecting the member to an anodic electrolyzing treatment (member becomes the anode) and a cathodic electrolyzing treatment (member becomes the cathode) in an electrolyte bath containing chloride ions for the purpose of removing the oxide layer; (b) rinsing the member; (c) strike plating the member; and (d) electroplating the struck member.
U.S. Pat. No. 4,938,850 discloses a method for plating electroless nickel onto a piece of titanium consisting of the steps of: (a) cleaning the piece of titanium; (b) contacting the piece of titanium to a concentrated hydrochloric solution; (c) activating the piece of titanium in a solution of nitric acid and hydrofluoric acid to remove the oxide layer; (d) treating the surface of the piece of titanium by ‘anodic processing’ in a treatment solution of acetic acid and hydrofluoric acid to avoid the formation of an oxide film on the titanium; (e) rinsing the piece of titanium; (f) strike plating the piece of titanium; (g) electroless plating the piece of titanium with a nickel layer; and (h) heat treating the piece of titanium.
The '524 patent and the '850 patent both teach removal of the oxide layer from the piece of metal in an electrolyte bath, intermittently rinsing the surface of the metal with water, and subsequently striking the surface of the metal with nickel in a different electrolyte bath. In both methods, the metal is exposed to oxygen as it is physically lifted from the initial bath, rinsed, and placed into the next bath. This exposure to oxygen promotes the growth of an oxide layer on the surface of the metal prior to the step of strike plating the metal with nickel. This tends to result in poor adhesion between the metallic coating and the metal.
There is an ongoing need for a method of surface treating a titanium-containing metal to remove the oxide layer and subsequently plating the treated surface of the metal with an adherent metallic coating before the reformation of the oxide layer.